Diffuse X-ray Emission from the Local Galaxy

Photo of Diffuse X-ray Emission from the Local Galaxy
  • Project Type: Mission
  • Acronym: DXL
  • Class: Flight Project

The Diffuse X-ray Emission from the Local Galaxy (DXL) payload utilizes an old payload repurposed for new science. The interstellar medium in the solar neighborhood is dominated by a neutral hydrogen cavity that extends ~50 light-years from the sun in the galactic plane and up to 500 light-years toward the galactic poles. The solar system currently lies within a diffuse, partially ionized cloud of limited extent, but for many years most of the cavity was thought to be filled with million-degree plasma. This came to be known as the Local Hot Bubble and was thought to be the origin for most of the observed 1/4 keV diffuse soft X-ray background (SXRB).

In the early 2000s, this picture was called into question by the discovery that the solar system itself was a source of diffuse X-rays. It was suggested that solar wind charge exchange (SWCX) could be responsible for all of the X-rays originating in front of the bounding galactic neutral hydrogen that defined the Local Cavity. In order to resolve this controversy, ASD scientists devised DXL to determine the SWCX flux by observing a feature resulting from the flow of interstellar gas though the solar system: the helium focusing cone, an enhancement downstream of the sun caused by gravitational focusing. This led to a proposal to map the SXRB by flying proportional counters built at the University of Wisconsin (supported by NASA grants) in the 1970s on a sounding rocket.

The DXL payload contains two 800 cm2 proportional counters built in the late 1970s to map the soft X-ray background. However, DXL uses the same large-grasp instrument to spatially disentangle the heliospheric charge exchange emission from more distant sources of soft X-ray emission, such as the Local Hot Bubble, and compare the results to modern spatial and temporal models of solar wind charge exchange.

The DXL payload also includes a Goddard-provided instrument, STORM, as a technology demonstration. STORM is a microchannel-plate X-ray detector with a wide field of view (10 × 10 degrees) lobster-eye slumped microchannel-plate optic. This is a prototype instrument for a full-scale magnetospheric charge exchange X-ray imager, and DXL is the first space-flight demonstration of this technology.

The instruments were refurbished at the University of Miami. The DXL and STORM payloads successfully flew on December 12, 2012 on a NASA Black Brant IX sounding rocket launched from White Sands Missile Range. Both instruments performed flawlessly and landed without damage. DXL clearly observed charge exchange from interplanetary helium in the helium focusing cone and fully constrained the charge exchange contribution to the Local Hot Bubble. When the data were compared to the ROSAT map of the SXRB, the team identified a clear SWCX signal from the helium focusing cone. Based on these results, the team determined that only about 40 percent of the observed flux in the galactic plane originated within the heliosphere. Because the walls bounding the Local Cavity are optically thick at 1/4 keV, the remainder of the observed emission must originate from inside it.

The DXL project brings together scientists from several disparate fields who have a strong interest in these phenomena. The PI institution is the University of Miami, with the University of Kansas, the University of Wisconsin, and Leicester University as collaborating institutions. The next flight of DXL will add a third energy band proportional counter and is scheduled for December 2015.

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